1. Field of the Invention
This invention relates to compact zoom lenses, and, more particularly, to zoom lenses of a varifocal ratio as high as 4 which enable a good compromise between the requirements of increasing the varifocal ratio and of limiting the overall dimensions of the lens to a minimum to be achieved while maintaining good stability of aberration correction throughout extended zooming ranges.
2. Description of the Prior Art
Zoom lenses which have been most commonly used include the so-called 4-component zoom lenses which have been proposed in Japanese Laid-Open patent applications Nos. SHO 51-37247, 51-63635 and 53-131852.
This 4-component zoom lens is constructed from four lens components of which the first, counting from front, remains stationary during zooming but is axially movable for focusing, the second is axially movable for varying the focal length of the entire system, the third is axially movable for maintaining the constant position of an image plane as the aforesaid second component moves axially to effect zooming, and the fourth remains stationary during zooming, and is used for satisfying determination of a desired longest or shortest focal length and a desired point at which an image of an object is formed.
Since, in such configuration, all duty for increasing the magnification power is given only to the second lens component, for the range of variation of the focal length of the entire system to be extended, measures must be resorted to by strengthening the refractive power of the second lens component, or by increasing the total axial movement of the second lens component. As the refractive power of the second lens component increases, good correction of aberrations becomes more difficult to achieve. Also, the increase in the total movement of the second lens component calls for an increase in the physical length of the zoom lens, thus sacrificing the compactness.
Attempts have been made to eliminate the abovedescribed drawback of the conventional zoom lens by imparting a forward axial movement to the first lens component which was heretofore held stationary when zooming so that the magnification power changing effect of the second lens component is increased, as, for example, disclosed in Japanese Laid-Open patent applications Nos. SHO 53-34539, SHO 54-25747, SHO 54-26754 and SHO 54-30855. However, this method is, because of its use of three lens components for zooming, associated with a very complicated calculation for determining the relation in which the three lens components are moved differently from each other. Also, because the number of parts constituting an operating mechanism for the zoom lens components is increased, the complexity of structure of a mechanical mounting for the zoom lens is increased. This method also is not well suited to reducing the bulk and size of the entire system.
There is also known the 2-component zoom lens comprising a negative front component and a positive rear component, both of which are moved axially but in different relation to effect zooming. Since this type is, however, difficult to achieve a higher zoom ratio, the maximum possible value was limited to about 2.